The helical systems of recording digital data on magnetic tape include a fixed head system which forms recording tracks in the longitudinal direction on magnetic tape by means of fixed heads and a helical scan system which forms recording tracks slantwise with respect to the longitudinal direction of the magnetic tape by means of rotary heads. The helical scan system is useful for high density recording and high speed data transfer.
When data is recorded onto the magnetic tape, the data is not written verbatim because of the importance of data integrity. Instead, additional characters are calculated and added to the data as it is written. This process, known as Error Correction Coding (ECC), allows the original data to be reconstructed at a later date even if a small number of data bits become damaged and unreadable.
When the helical system is instructed to read a certain file, the system positions the magnetic tape at the beginning of the file of interest and begins reading the data. The ECC is used to check the integrity of the data and correct any errors that are very short in duration. The system will stop reading the data and rewind to the beginning of the error region when the error is too long in duration for the ECC to correct. Then the system will attempt to reread the data in the error region. Most read errors are caused by debris contaminating the read heads. The debris is often dislodged during the reread attempt which makes the data readable. The reread operation is attempted again if the data remains unreadable. The system will quit after a certain number of retries and indicate that the data is unrecoverable.
Sometimes all the data, or a large fraction of the data, becomes unreadable for several seconds due to the fact that the recorded helical data tracks are curved rather than straight or are displaced from their specified locations. This occurs when the edge of the tape is damaged or when debris contaminates the guides in the tape path. Most practical error correction codes are only able to correct very short and intermittent data errors. A long error, however, is generally not correctable by ECC because the data tracks are not located where the tape drive attempts to read them.